Relaxin is a peptide hormone, structurally homologous to insulin, which is found in the corpora lutea of pregnant women. We have shown that relaxin obtained from human corpora lutea of pregnancy has biological activity in the guinea pig pubic symphysis assay and immunoactivity in a procine relaxin RIA. Immunoactive relaxin is present in the sera of pregnant women from two weeks after conception until delivery. In preliminary study, we have demonstrated that human relaxin decreases the amplitude of spontaneous contractions of human myometrial strips in vitro. The objective of the current study is to first determine the normal secretion patterns of relaxin all part of pregnancy, labor and the puerperium. Induced and spontaneous labors will be compared. Serum relaxin levels will also be determined in patients with incompetent cervices, premature labor, postmature pregnancies and spontaneous abortion. The effect of exogenous hCG on the induction of relaxin secretion by luteal phase women will be studied. Using an in vitro human myometrium strip, the effect of human relaxin in spontaneous myometrial contractions will be quantitated. The effects of the physiological substances which after myometrial contractions will be studied and the possible synergistic or antagonistic actions of human relaxin will be determined.